text: //------------------------------------------------------------------------------
  /*
      This file is part of wrtd: https://github.com/World-of-Retail-Token/wrtd
      Copyright (c) 2019 Ripple Labs Inc.
      Copyright (c) 2019 WORLD OF RETAIL SERVICES LIMITED.
  
      Permission to use, copy, modify, and/or distribute this software for any
      purpose  with  or without fee is hereby granted, provided that the above
      copyright notice and this permission notice appear in all copies.
  
      THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
      WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
      MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
      ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
      WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
      ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
      OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
  //==============================================================================
  
  #ifndef RIPPLE_BASICS_LOCALVALUE_H_INCLUDED
  #define RIPPLE_BASICS_LOCALVALUE_H_INCLUDED
  
  #include <boost/thread/tss.hpp>
  #include <memory>
  #include <unordered_map>
  
  namespace ripple {
  
  namespace detail {
  
  struct LocalValues
  {
      explicit LocalValues() = default;
  
      bool onCoro = true;
  
      struct BasicValue
      {
          virtual ~BasicValue() = default;
          virtual void* get() = 0;
      };
  
      template <class T>
      struct Value : BasicValue
      {
          T t_;
  
          Value() = default;
          explicit Value(T const& t) : t_(t) {}
  
          void* get() override
          {
              return &t_;
          }
      };
  
      // Keys are the address of a LocalValue.
      std::unordered_map<void const*, std::unique_ptr<BasicValue>> values;
  
      static
      inline
      void
      cleanup(LocalValues* lvs)
      {
          if (lvs && ! lvs->onCoro)
              delete lvs;
      }
  };
  
  template<class = void>
  boost::thread_specific_ptr<detail::LocalValues>&
  getLocalValues()
  {
      static boost::thread_specific_ptr<
          detail::LocalValues> tsp(&detail::LocalValues::cleanup);
      return tsp;
  }
  
  } // detail
  
  template <class T>
  class LocalValue
  {
  public:
      template <class... Args>
      LocalValue(Args&&... args)
          : t_(std::forward<Args>(args)...)
      {
      }
  
      /** Stores instance of T specific to the calling coroutine or thread. */
      T& operator*();
  
      /** Stores instance of T specific to the calling coroutine or thread. */
      T* operator->()
      {
          return &**this;
      }
  
  private:
      T t_;
  };
  
  template <class T>
  T&
  LocalValue<T>::operator*()
  {
      auto lvs = detail::getLocalValues().get();
      if (! lvs)
      {
          lvs = new detail::LocalValues();
          lvs->onCoro = false;
          detail::getLocalValues().reset(lvs);
      }
      else
      {
          auto const iter = lvs->values.find(this);
          if (iter != lvs->values.end())
              return *reinterpret_cast<T*>(iter->second->get());
      }
  
      return *reinterpret_cast<T*>(lvs->values.emplace(this,
          std::make_unique<detail::LocalValues::Value<T>>(t_)).
              first->second->get());
  }
  } // ripple
  
  #endif
  